Private branch exchange telephone system including conference facilities



A. E. GERBORE ETAL PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM July 30, 1963 INCLUDING CONFERENCE FAcILxTIEs 13 Sheets-Sheet 1 Filed Dec. 8', 1961 ATTOR/VV July 30, 1963 A. E. Gl-:RBORE ETAL 3,099,719

PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM INCLUDING CONFERENCE FACILITIES 13 Sheets-Sheet 2 Filed Dec. 8, 1961 July 30, 1963 A. E. GERBORE ETAL PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM INCLUDING CONFERENCE FACILITIES Filed Dec. 48, 1961 13 Sheets-Sheet 5 QQQQ QQ QQkQ ood, Q3 A Nv mi@ Bv A. E. GERBORE ETAL PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM INCLUDING CONFERENCE FACILITIES July 30, 1963 F'iled Dec. 8, 1961 July 30, 1963 A. E. GERBORE ETAL. 3,099,719

PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM INCLUDING CONFERENCE FACILITIES 15 Sheets-Sheet 5 Filed Dec. 8. 1961 Nd bti www July 30, 1963 vA. E. GERBORE ETAL 3,099,719

PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM INCLUDING CONFERENCE FACILITIES 13 Sheets-Sheet 6 Filed Dec. .8, 1961 July 30, 1963 A. E. GERBQRE ETAL 3,099,719

PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM INCLUDING CONFERENCE FACILITIES 13 Sheets-Sheet 7 Filed Dec. 8, 1961 July 30, 1953 A. E. GERBORE ETAL. 3,099,719

PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM INCLUDING CONFERENCE FACILITIES 13 Sheets-Sheet 8 Filed` Dec. 8, 1961 Lil@ A. E. GERBORE ETAL PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM .July 30, 1963 3,099,719 INCLUDING CONFERENCE FACILITIES l5 Sheets-Sheec 9 Filed Dec. 8, 1961 July 30, 1963 A. E. GERBORE ETAL PRIVATE BRANC Filed Dec. 8, 1961 15 Sheets-Sheet l0 Kku 05 D M NEEN S mi July 30, 1963 A. E. GERBORE ETAL PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM INCLUDING CONFERENCE FACILITIES I Filed Dec. 8. 1961 13 Sheets-Sheet 1l Ok kbk LES N July 30, 1963 A. E. GERBORE ETAL 3,099,719

PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM INCLUDING CONFERENCE FACILITIES 13 Sheets-Sheet 12 Filed Dec. 8, 1961 July 30, 1963 A. E. GERBORE ETAL 3,099,719

PRIVATE BRANCH EXCHANGE TELEPHONE SYSTEM INCLUDING CONFERENCE FACILITIES v Filed Dec. 8, 1961 13 Sheets-Sheet 13 l. CESI. 3.3i TS1 N271 B l. NCBI.; N811. NT3. @1Km mvNQ @IEEE I@ I@ E E Q mE miv miv United States Patent O York Filed Dec. 8, 1961, Ser. No. 158,027 8 Claims. (Cl. 179-27112) This invention relates to a private branch exchange telephone system and more particularly to a private branch exchange telephone system which includes conference facilities.

In instances where a telephone customer, for example a business firm, a Government agency, or the like requires a relatively large number of extensions, it is the usual practice to provide on the premises, or conveniently located thereto, a private branch exchange commonly referred to as a PBX. The larger PBXs are usually served by one or more attendants from a console or attendants position and function essentially as a small telephone central office in that intrastation calls between extensions on the premises are completed through the PBX as well as calls from the extensions to subscribers served through remote central oilices and calls from such subscribers to extensions served by the PBX. A PBX of a type `capable of serving a relatively large number of extensions and providing many advanced features is disclosed in Patent 2,904,637, issued September 15, 1959, to R. D. Williams.

It is fairly common practice to provide in connection with the PBX, conference facilities by means of which several of the extension stations may be connected together in a common loop for communication with each other and, on occasion, such a conference setup may be connected to an outside line through the PBX. It is desirable, of course, that subscribers joined in these conference calls be provided the same high quality transmission as that prevailing in the instance f two-party connections and, further, that the provision of the conference facilities does not interfere with or reduce the availability of the lines for ordinary connections.

Accordingly, it is an object of our invention to improve the operation of private branch exchange telephone systems.

Another object of the invention is to enhance and improve conference facilities associated with a private branch exchange.

A still further object of the invention is to visually identify lines associated with a conference connection at any given moment.

In accordance with a specific embodiment of our invention, wherein a plurality of four-wire lines may be interconnected as desired under control of an attendant, two conference bridges are provided each having a capacity of ve lines. Independent conference connections can be set up on the two bridges under control of the attendant, each conference connection having a maximum capacity of iive lines; the respective conference bridges can be connected to outside lines if desired. In accordance with the novel arrangement contemplated by the invention, the two conference bridges can, on occasion, be connected in tandem whereby to provide a single conference connection with a capability of ten lines.

`A feature of our invention is resistance bridges used for connecting lines in a conference loop whereby to attain high quality transmission substantially free of echo effects.

A further feature of the invention is means for connecting desired lines to each of a pair of conference bridges and to connect said bridges in tandem whereby to provide a common conference connection of relatively large capacity.

A still further feature of the invention is means effective upon disconnection from either respective tandemlyconnected bridge of all the station lines connected thereto to interrupt said tandem connection and restore the respective bridges to independent status.

Yet another feature of our invention is means controlled by a single key operation and effective at any time while a conference is in progress for identifying all lines associated with a conference bridge. 1 =1`A full understanding of the arrangement contemplated by the present invention as well as an appreciation of the various features thereof may be gained from consideration of the following detailed description in connection with the accompanying drawing, in which:

FIG. 1 shows schematically the relationship of the various individual circuits which comprise one specilic illustrative embodiment of the private branch exchange system contemplated by the invention;

FIG. 2 shows the four-wire auxiliary line circuit;

FIGS. 3 and 4 show one line circuit;

FIG. 5 shows schematically portions of the link and connector circuit as shown and described completely in the Williams Patent 2,904,637 referred to above;

FIGS. 6 and 7 show particularly the conference line circuits, the rst conference line circuit being shown in detail and the second being indicated by a captioned rectangle;

FIGS. 8 and 9 show the auxiliary line circuit used with E and M lead signaling;

FIG. 10 shows the line pickup circuit;

FIG. 11 shows the two-way, two-wire trunk circuit for connecting to a central office;

FIGS. 12 and 13 show the attendants position and console circuits; and

FIG. 14 shows the manner in which certain'of the iigures should be arranged to show the specific illustrative embodiment of the invention.

General Description The arrangement and operation of the various components of the illustrative embodiment of the invention will be described in detail subsequently with reference to FIGS. 2 to 14, inclusive. However, in order to first gain a general overall understanding of the arrangement contemplated, a brief general description will be given at this point with reference to FIG. 1. Referring, therefore, to FIG. 1 it will be noted that the two groups of four-wire extension stations, and the central office are all connected to the PBX over different paths, all of which include similar line circuits 121, 122 .and 123` (FIGS. 3 and 4) but each of which includes a different type of intermediate circuit. For example, the first group of four-wire extension stations is connected through four-wire auxiliary line circuit 124 (FIG. 2). This circuit, which will be described in detail subsequently, recognizes off-hook and on-hook signals from the station and passes them along to line circuit 121, recognizes an incoming call from line circuit 121 and applies machine ringing current to the station loop, and recognizes the answer condition at the station and removes the ringing current from the loop.

Another group of four-wire stations is connected to the associated line circuit |122 through auxiliary line circuit Patented July 30, 1963 128 which is used with E and M signaling. Circuit 128 provides functions similar to that of auxiliary line circuit 124 by way of carrier or other facilities utilizing E and M lead signaling.

The central oce connection is made over a twowire line to twoway trunkcircuit 11311. This circuit functions to recognizeseizure from the central office and to pass this seizure along to line circuit `1123, to recognize seizure from line circuit 123 and to pass this seizure along to the central office, and to provide a hybrid arrangement whereby to convert from four-wire to twowire operation.

Respective line circuits l121, 122 and 123 each have three access points. Toward the lines it connects to an auxiliary line circuit (circuits 124, i128, or.r131) which receives line signals and translates them for use in the line circuit. Toward the automatic switching mechanism it connects to the links of link and connector circuit 132. Access from the links is used for terminating calls to this line circuit and to extend calls to other lines from this circuit. A thirdvaccess is provided for the attendant, i.e., to attendants position 'and console circuits 133. Thisl access is provided with call progress lamps and a busy lamp. It is used by the attendant to answer incoming calls and to extend these calls to other lines by way of thelinks. It is also used to gain access to any line at any time, whether busy or idle, for originating a call, answering a call, disconnecting an established call or monitoring. Some of the general functions of each line circuit are to recognize seizure incoming and to signal the attendant, to recognize seizure outffoing and to light a busy lamp, to recognize seizure by the attendant and light a busy lamp, to make itself busy when seized by the attendant or the automatic switching circuits, and to exchange supervision between connected line circuits and the attendant.-

Line pickup circuit :134 is included in the path over which the line circuits are connected to the attendants position and console circuits .133; the circuit functions to connectv leads of a respective line circuit to the position circuit, to prevent the simultaneous connection of the position circuit to more than one line circuit, to prevent two position circuits from being connected to the same line circuit, and to release the connection when a s-ubsequentline isselected or upon receipt of a release signal from the attendant.

The functions of the attendants position and console circuits have .been briefly discussed above in connection with the line circuits. In general, the console circuit provides the keys, lamps, jacks, headset, handset and dial for the attendants. The design of the console per se may be as disclosed in United States Patent Des. 193,513, issuedSeptember 4, 1962, onthe applicationof F..P. Wight, Serial No. 67,038, led October l0, 19611. The position circuit includes a suitable four-wire resistance bridge arrangement to permit a three-way conversation while maintainingadequate transmission levels.

Among the signal lamps included in the console circuit are a Source lamp for each line and a Destination lamp, also one for each line. The source lamps are the supervisory lamps for the calling stations and the destination lampsv are the supervisory lamps for the called stations. The operation of typical source and destination lamps will: be described subsequently in connection with the detailed circuit description. The indications given by each areas follows.

Destination lamp indications:

(l) Dark-connection to the called station has not been established or the station has gone on-hook.

(2) 30 i.p.m. lash-awaiting olf-hook signal from called station to line.

(3) Lighted steady-called station is off-hook.

(4) 120 i.p.m. ash-called station recalling attendant.

As mentioned above and as will be described in detail subsequently, the arrangement contemplated by our invention includes novel conference facilities. The conference line circuits and associated conference bridges, No. 1 and No. 2 are indicated by rectangle 137. Each bridge is provdied with six legs, rlive of the legs being provided for connection of selected lines to the respective bridge and the sixth leg of each bridge being utilized on occasion for interconnection of the two bridges in tandem. Each of the conference bridges, No. 1 and No. 2, comprises a six-leg resistance bridge of the general arrangement disclosed in H. A. -Rhodes Patent 2,755,445, issued July 17, 1956. When the atendant has ascertained the station lines to be associated with a conference, she will first originate a call to the first line, in a manner to be described in detail subsequently, and then will by direct station selection connect the line to one leg of the conference line circuit. This action will be repeated until all the desired lines (five or less) have been connected to the respective bridge.

The two bridges may be used independently and will ordinarily be so used so long as neither conference includes more than ve lines, the maximum capacity of each bridge. ln the event a larger conference (up to ten lines) is desired, the novel arrangement provided is such that by a single key operation by the attendant the two bridges may be connected in tandem whereby to permit a single conference connection with a capability of ten lines. Substantially distortionless transmission is attained for the conference circuit through use of the two balanced resistance coupling structures and the novel utilization of one of the legs of each for the interconnection of the two. By a further novel means, the tandem connection of the two bridges is automatically interrupted if all lines connected to either bridge go on-hook and the two bridges are restored to independent status. Also, during the conference connection all lines associated with a conference bridge, or bridges, can be identied by a single key opera-tion by the attendant.

Bt will be apparent from the above brief description, and still more apparent from the subsequent detailed description, that a novel and effective conference arrangementA is provided whereby a conference connection of relatively large capacity may be established on occasion, kand yet where the-flexibility and other advantages of smaller capacity, individual conference connections are not permanently sacriiiced.

It will be apparent, of course, that when the two bridges have been connected in tandem, neither should be permitted to hang idle, that is, if all the lines connected to either bridge go ron-hook the tandem connection should be interrupted without undue delay and the bridges restored to independent status so that the idle bridge may be available for further use. means referred tto above accomplishes this desirable objective entirely automatically and without any monitoring actions [on the part of the attendant.

Direct Station Selection referred to yabove is an arrangement recently provided in centain PBX installations whereby the attendant may complete a call to an extension station merely by operation of the DSS key for that station, maintaining it operated until the associated DSS lamp lights, `and then releasing it. Operation of the DSS key generates the 'tens and units information corresponding to the called `extension and transmits the information to the dial pulse register in a man-` The restoring ner similar to that prevailing in dial operation. One type of direct station selection is disclosed in Un-ited States Patent 3,050,588, issued August 21, 1962, on the application, Registration Control Circuit, Serial No. 645, led January 5, 1960, M. L. Benson and C. Breen.

Incoming Call From Extension Station Referring now to FIGS. 2 to 13 of the drawing, a more detailed description of the contemplated arrangement will be given. It will be noted that the illustrated system utilizes a four-wire line with transmit and receive loops and that, at points where the lines are coupled by repeat coils, simplex paths `are obtained `for signaling and supervision purposes by midcoil connections in the wellknown manner. It will be assumed rst that an incoming call originates at a four-wire extension station connected to four-wire auxiliary line circuit 124 (FIGS. 1 and 2). When the extension station (not shown) originates a call by going oit-hook, the closed 'IT- TR loop operates relay 2L over an obvious path which includes leads 251 and 252 and the two windings of the relay. Talking battery is supplied to the station from relay 2L over leads 251 and 252. Relay 2L, operated, completes through its 2L-1 make contact an obvious operating path for relay 2L1. Upon operation of relay 2L1, ground is connected through its 2L1-1 make contact to lead 253 as an ofi-hook signal to the line circuit.

Relay 2L1, operated, also completes through its 2L1-21 make contact lan operating path `for rel-ay 2L0y (FIG. 2). Relay 2L0, upon operating, partially completes a locking path through its 2L0-1 make contact and opens at its 2L0-2 break contact the operating path of relay 2SR. Relay 21.0, operated, also connects through its 2L0-3 and 2L0-4 make contacts the windings of relay 2L2 to the T-R station receive loop, leads 254 and 257.

The ground `applied to lead y253 by operation of relay 2111, as `above described, operates relay SLL (FIG. 3) over an obvious path which includes break contact 3RV-1 of relay 3'RV. Operation of relay 3LL in turn completes through its make contact 3LL-1 Ian operating path for slow-release relay 3LL1. With relay 3LL1 operated, interrupted battery is applied through inter-rupter cont-act INT-1 (FIG. 4), make contact 3LL1-1 of relay 3LL1, break contacts 4AC-1 of relay 4AC and y4ACA-1 of relay 4ACA to source lamp lead 458 (SL00) to source lamp 1259 (S00) at the attendants position (-FIG. 12); lamp 1259 is flashed at 120` i.p.m., the rate at which the battery is interrupted, whereby to attract the `attention of the attendant to the incoming call.

It will be understood, :of course, that there is an individual source lamp (as well as individual destination lamps, pickup lamps and so on) for each line served by the PBX and that individual control paths are provided for each. However, for purpose of simplification and clarity, there will be actually shown only typical ones (usually the rst and last of the series) Yof these duplicated elemenlts, and dot-dash lines with brackets will be utilized in an obvious manner to indicate the simulation of a group of similar leads and yof the carrying over of such groups lfrom one point to another.

Also, relay 3-LL1 operati-ng as `above described, connects negative potential to lead 461 (DK00) over its 3LL12 make contact and Ibreak contact 1H-41 of relay 4H whereby to light direct station selection lamp 1262 (PDSS00) at ste-ady, lamp 1262 being located at the attendants position (FIG. 12). Relay 3LL1, upon operating, also connects ground through its make contact 3LL1-3 (FIG. 3) `a-nd break contacts 4LH-1 of relay 4LH, 4ACA-2 of relay 4ACA, 4L1D-1 of relay 4L1D, 4H-2 of relay 4H and 4ATI`1 and y4ATI`2 of relay 4A'IT to lead 363 Iand thereover to battery on the winding of relay i13SIG (FIG. l13) of fthe console circuit. Operation of relay 13SIG causes an audible signal to sound as will be described subsequently.

Attendant Responds to Call The attendant, being `alerted by the audible signal referred to and the flashing source lampy -1259 (FIG. l2) to the call originated at the extension station, operates the particular pickup key in the console circuit for the calling line; this causes the line pickup circuit (FIG. 10) to function in the following manner to connect the attendant to this line. Ground through fthe particular pickup key involved, here taken to be the rst key PU00, operates relays -10RL and 1080 in series over an obvious path which includes break contact 10RL1-1 of relay 10RL1. Relay 1050, operated, completes a path for operating the 10SMO select magnets of the line pickup crossbar switch from battery, winding of magnet V10'SM0, make Contact 1080-2 of relay 10S0, break contacts 1089-1 of relay 1089, 1052-41 of relay 10S2, and 10S1-1 of relay 1081, make contact 1050-3 of relay 1080 to ground. Relay 10RL, operated closes an operate path for relay 13GS (FIG. 13) from battery, winding of relay ISGS, make contact 10RL-5 of relay 10RL to ground; relay I13GS upon operating completes a path for operating relay I10TR (FIG. 110) from ground, make contact ISGS-1 of relay ISGS, break contact 10TS-1 fof relay 10I`S, diode 1067, thermistor 1068, winding of relay 10fI'R to battery. Relay 10TR upon operating releases (at break contact 10TR-4) any hold magnets which may be operated at this time and completes a path for operating relay 10TS traced from ground, make contact 13GS-1 of relay ISGS, make Contact 10TR-2 of relay 10T R, winding of relay =10TS to battery; relay 10TS upon operating locks to ground through its own make contact -10TS-2 and make contact 13GS-1 of relay 13GS, and at this time interrupts at its break contact 10I`S-1 the operate path for relay 10TR which releases.

With relay 10TS operated :and relay 10TR released, relay 10RL1 (FIG. 10) operates over a path from ground, make contact 10TS-3 of relay 10TS, break contact 10TR-3 :of relay 10TR, make contact NRL-1 of relay 10RL, break contact 10R-L1-6 and winding of relay IORL1 to battery; relay 10RL1 upon operating locks to ground through its own make contact RLI-Z and make contact -10S0-1 of relay 10S0.

Relay 10RL1 operated interrupts at its 10RL1-1 break contact the operating path for relay 10RL, which releases,

but establishes at the same time at its 10RL1-4 make contact a path for holding relay 1080. The chai-n circuits through the contacts of the 1080 to 1089 relays check that one, and only one, select magnet (10SMOL- 10SM9) of each switch has ground connected to it. The same select magnet in each switch will operate; the select magnets and the *10RL1 relay will remain operated as long as the pickup key in the console is held operated.

Also at the `above-referred-to point, that is with relay 10TS operated and relay 10TR released, an operate path for hold magnet 10IIMO (FIG. 10) is completed from battery, winding and break contact 10HMO*1 of hold magnet 10'I-IMO, make lcontact of operated pickup key PU00, break contact 10CK-1 of relay 10CK, make contact 10SMO-1 of select magnet 10SMO, break contact 10TR-3 of relay 10TR, make contact 10TS-3 of relay 10TS to ground. Hold magnet |10HMO, upon operating, locks to ground through its 10IIMO-2 make contact and break contact 10TR`4 of relay 10TR, completes through its HMO-3 make contact an operating path for relay 10CK (FIG. 10) and closes one set of crosspoints of the pickup switch. v

Closure of the crosspoints of the pickup switch is effective to connect the twelve leads 381, 382, 383, 384, 387, 388, 391, 392, 393, 394, `397, and 39'8 from the line circuit associated with the originated call (FIG. 3) through the line pickup ycircuit (FIG. 10) to the attendants position and console circuits. Here, as in previouslyreferred-to instances, in order to avoid undue complication of the drawing, dot-dash lines and brackets have 7. been utilized in an obvious manner to indicate the transfer lof this group of leads from one point to anotler.

When the position circuit has been connected to the originating line as above described, ground is connected to lead 398 (FIG. 12) through break contact ISGS-2 of relay 13GS, relay ISGS having released following release of relay IGRL. Relay iAC (FIG. 4) of the line circuit operates from this ground on lead 398 through break contacts `EH-3` of -relay 4H, `4AC2 of relay i-AC, and 4ACA-3 of relay 4ACA, winding of relay 4AC to battery; relay 4A() upon operating locks to ground through -its 4AC-3 make contact.

Relay 4AC, operated, also completes through its 4AC-4 make contact an obvious operating path for relay 4L1D (FIG. 4), interrupts at its 4AC-1 break contact (FIG. 4) the connection of interrupted battery to lead 458 (SL) and connects uninterrupted battery thereto through make contacts SLL-2 of relay SLL, 3LLI-4 of relay 3LL1, and 4AC-5 of relay `4AC, break contact 4ACA-1 of relay 4ACA, lead `458 to source lamp 1259 (S00) of the position circuit (FIG. 12); this changes lamp 1259 from ashing to steady as indication that the attendant has answered the call. Also, relay 4AC, operated, opens the transmission paths at its break contacts 4AC-5, 4AC-6, 4AC-7 and AC-S (FIG. 3) whereby to split the line since `it has now been connected to the attendants telephone set bridge in the position circuit.

At this point relay 3S (FIG. 3) operates from the simplex ground at the attendants position circuit (FIG. 12) over leads 4381 -and 382; relay 3S, operated, applies ground through its 3S-1 make contact (FIG. 3) to lead 311 and operates relay ZAPL (FIG. 2) for a purpose described below.

Relay 4L1D, opera-ted as above described, interrupts at its 4L1D-1 and 4L1D-2 break contacts (FIG. 3) the paths for connection of ground to lead 363 and relay 13SIG (FIG. 13) which controls the audible signal at the attendants position is accordingly released. Also, relay 4L1D upon operating connects battery through its 4L1D-3 make Contact to lead 4512 (PKW), whereby to light pickup lamp 11213 (PUtitl) steady at the console, and connects ground through its make contact 4L1D-S `and break contacts 4DSS-1 or relay 4DSS and 4ATT-3 of relay 4A'IT to lead v414 and thence to the marker 517 (FIG. for making the line busy to the marker in the usual manner.

When the answer signal operates relay ZAPL (FIG. 2) las above described, operation of relay 2APL1 follows on the path from ground, make contact ZAPL-ll of relay ZAPL,` break contact 2APL2-1 of relay ZAPLZ, winding lof-relay 2APL1 to battery; relay ZAPLI upon operating locks through its ZAPLl-l make contact and make contact 2APL-1 of relay APLl to ground, and opens at lits 2APL1-2 break contact the operating path of relay 2RR whereby to prevent the operation of that relay upon `operation of relay 2APL2. Relay 2APL2 operates following operation of relay ZAPLI, the operating pathbeingcompletod through cake contact 2APL1-3 of that relay `and locks to ground through its own make contact 2APL22 and make contact 2L1l-2 of relay 2L1. Relay 2APL2, operated, partially completes through its make contacts 2APL2-3 and 2APL2-4, respectively, operating paths for relays ZRR and 2SR, which have no circuit functions at this time, and also completes through its make contact 2APL2F5 and make contact ZL-l of relay 2L() a locking path for relay 2L@ whereby to prevent relay 2L() from releasing before the release of relay 2APL2 lat the time of disconnect. Relay ZAPLZ, operated, also opens atits 2APL2-1 break contact the operating path of relay 2APL1 which however remains. locked through its own make contact 2APL1l-1.

The circuits are now in condition for conversation 8, between the originating extension station and the atfdendant.

Attendant Extends Connection After determining from the originating subscriber the identification of the desired called line, the attendant Signals for connection of the line circuit to a register by operation of the PDSS (Direct Station Selection) key of the console circuit `(FIG'. 13) which corresponds to the desired called line; it will be assumed that PDSSG key is operated.

Relay 13TM of the position circuit (FIG. 13) is in operated position at this time following closure of the Talk key 1382 when the attendant picked up the line (ground, make contact MICK-2 of relay ItlCK, make contact of Talk key 11382, winding of relay 13TM to battery) so that upon closure of PDSSEE()` key las abovementioned relay 13PDSS (FIG. 13) operates over a path from ground, make contact 13TM-1 of relay ISTM, closed contacts of PDSSlizti key, winding fof relay ISPDSS to battery; operation of relay ISPDSS applies ground through its 13PDSS-1 make contact (FIG. 12) to lead 397 whichcauses operation of relay `IDSS (FIG. 4) of the line circuit.

Relay 4DSS, operated, connects ground through its maker contacts 4DS1S-1 and 4DSS-2 and make contact 3LL1-4 of relay 3LL1 to leads 418 and 421 and thereover to marker 517 which responds by selecting an idle link, an idle register, by identifying the calling line and by connecting the line to the register over the selected link. (As previously mentioned these portions of the circuit are in arrangementand operation generally as described in previously issued patents, for `example R. D. Williams Patent 2,904,637, and willv not be described in detail herein.)

'Relay 4DSS, operated, also removes at its 4DSS-11 break Contact (FIG. 4), the make busy ground from lead 414 whereby to allow access by marker 517 to the line, and connects ground through its 4DSS-4 make contact (FIG. 3) to line 311 whereby to retain answered supervision to the auxiliary line circuit.

Marker `517 in line with the `above-referred--to operations applies ground to line `414 as a signal .to the line that the link and register have been selected; this ground on lead 414 is extended through break contact 4ATT-3 of relay 4AIT and make contacts 4L1D-5 of .relay 4L1D and 4DSS-5 of relay 4-DSS, ythrough the winding of relay 4LH to battery whereby to operate relay 4LH. Relay 4LI-I, upon operating,locks to groundthrough its own make co,ntact-\4LH2, break contacts 4ATI`4 of relay 4ATT and 3RV-2-of relay SRV, make` contact 4L1D-6 of relay 4L1D to ground. Relay 4LH, operated, connects ground through its lLH-S make contact to lead 422 for operation of the hold magnet SLI-IM() (FIG. 5 associated with this particular line and extends `its holdingground (make contact lHAD-6 of relay 4L1D, FIG. 4) through its make contact 4LH-4 and break contact 4AT1`5 of relay IATI over lead 423 and `through resistor 424 toward the link and connector circuit (FIG. 5); thisy ground is used to lock the magnets for both-the line circuit and the register from which it follows that the entire connection is under control of the calling line circuit. Also, relay 4LHnpon operating opens at its break contact 4LH-S the operating path of relay 4DSS (FIG. 4) which releases and removes the ground signal fromleadsz418 and 421 (FIG. 4); connects interrupted battery through interrupter contact INT-2 (FIG. 4), its own make contact 4LH-6 and break contacts SRV-3 ofrelay SRV and 3LS'2-1 of relay 3LS2 to line 427 and thereover to the` position and console circuits (FIG. 12) where it ilashes destination lamp 1228 (D00) at 30 i.p.m. (the rate at which the battery is interrupted) as an Awaiting Called Station Answer signal; and connects ground through its make contact 4LI-I-7 (iF-IG. 3) and break contacts 3LS2-2 of relay 3LS2 and 4151-2 of relay 4LS1 to line 311 (FIGS.

3 and i2) whereby to maintain answered supervision to the calling party.

When PD'SStltl key (FIG. 13) was operated as above described, in addition to operating relay 4DSS of the line circuit, it also transmitted 4(by normal functioning of Direct Station Selection as mentioned above), through contacts 1388 and 13S-9 of the particular key, tens and units identification of the called line to the dial pulse registers 543 (FIG. 5) as indicated by the dot-dash lines and bracket. (Each extension line circuit has a two-digit number assignment corresponding to its position in the crossbar switch ield and its appearances at the attendants console.) In response to the tens and units information the register, operating in its normal calls in the marker which proceeds to connect the called line circuit (similar to the calling line circuit described) to the link originally used to connect the calling line to the register. Following this operation the register is released from the connection.

The called line circuit (similar to the calling line circuit, FIGS. 3 and 4) extends the RT and RR simplex mark to the connected auxiliary circuit over a lead correspending to lead 311 of FIGS. 2 and 3; the auxiliary line circuit responds by signalling the called station in the normal manner. For purposes of simplifying and reducing the size of the drawing, the called line and auxiliary line circuits will not be shown separately but the completion of the call will be described by referring to FIGS. 2 and 3; it will be understood that in this instance relays 2L and 2L2 will be in released position since the loop at the called station is still open. The ground applied to the control lead of the called auxiliary line circuit, which we will assume for the present description to be lead 311, will operate relay ZAPL; operation of relay ZAPL causes operation of relays 2AP=L1 and 2APL2 as previously described. In this instance, however, relay 2L0 does not operate since relay 21,1 is in released position. When the interrupter closes, ground is connected through closed Contact INT-3 of the interrupter, make contact 2APL2-4 of relay 2APL2, break Contact 2L1-2 of relay 2L, winding of relay 2SR to battery, relay 2SR operating over this path.

Relay ZSR, operated, connects A.C.-D.C. audible tone from source 244 through break contact .2L-3 of relay 2L and its own make contact ZSR-l to line conductor 257 and connects ground through its make contact 2SR-2 to the other line conductor 254. At the same time, i.e., when relay ZSR operates, it connects A.C.-D.C. audible tone from source 247 through its make Contact 2SR-3 to line conductor 252 toward the calling end and connects ground through its make contact 2SR-4 to line conductor 251. As relay ZSSR follows the operation of the interrupter, the connection of the audible tones and ground will be intermittently completed and interrupted whereby to produce a ringing cycle on the called line. The cycles will be repeated until the station answers by going oli-hook or until the call is abandoned.

Called Station Answers When the called station answers and the station loop is closed the operation of the called auxiliary line circuit is similar to that described above in connection with the origination of a call. In this instance however, operation of relay 2L will remove at its 2L-3 break contact the A.C.-D.C. tone from source 244 and operation of relay 2L@ (following operation of relays 2L and 2L'1) will open at its break contact 2150-2 the operating path of relay ZSR which releases and opens the tone and ground connections to the lines.

lIt will he recalled from preceding description that following operation of relay 4LI-I destination lamp 1228 (D00) of the console circuit (FIG. 12) was lighted and continued to flash at 30 i.p.m. `as an awaiting answer signal to Ithe attendant. When the called party answers the called line circuit grounds the 'IT-TR `simplex through the link 'and this ground is sensed by relay 3S (FIG. 3) of the calling line circuit through break cont-act SRV-4 of relay 3RV. Relay 3S operates, connects ground to Iline 311 (FIGS. 2 `and 3) through `its 3S-1 make Contact to maintain answered supervision, and completes through its 3S-2 make contact, a path for operation of relay 3SA (FIG. 3). Relay 38A, operated, completes an operating path for relay 4LS1 (FIG. 4) traced from ground, make contact 3LL1-5 of relay 3LL1, make contact SSA-1 of relay 3SA, break contact 3LS2-3 of relay 3LS2, winding of relay 4LS1 to battery. Relay 4LS1, upon operating, locks to the same ground through its own make contact 4LS1-2 and make contacts 4L1D-7 of relay 4L1D and 3S A1 of relay 3SA. Relay 4LS1, operated, completes a path for operating relay 3LS2 (FIG. 3) from ground, make contact 4LH-7 of relay 4LH, make contact 4LS1-3 of relay 4LS1, winding of relay 3LS2 -to battery.

After both relays 4LS1 and 3152 have operated, the interrupted `battery is removed from lead 427 at break contact 3LS2-1 of relay 3LS2 (FIG. 4) and uninterrupted battery is connected thereto through make contact 4LS1-4 of relay 4LS1 and make contact 3LS2-4 of relay SL82; destination lamp 1228 (D00) at the attendants position circuit (FIG. 12) is now lighted steady therefore as a called party answer signal. Also relay 4LS1 operated partially prepares through its make contact 4LS1-5 (FIG. 4) an operate path for relay 4L1D from the links if this :circuit is seized for identification later.

Attendant Releases W'nen the attendant relases the console circuit from the line, for example by operation of the Position Release Switch 1331 (FIG. 13) ground is removed from lead 398 at break contact ISGS-2 of relay 13GS (FIG. 12), since relay 13GS operates from ground through the closed contacts of switch 1351; removal of this ground permits relay SACA (lFIG. 4) to operate from battery through resistor 448, Winding of relay 4ACA, make contact 4AC-9 of relay 4AC, make `contact 3LL1-5 of relay 3LL1 to ground; relay 4ACA upon operating locks to this ground through its 4ACA-4 make Contact. Relay 4ACA, operated, opens at its 4ACA-3 break contact the holding path 0f relay LSAC which rele-ases; continues through its 4ACA-5 make contact (FIG. 4) the path for lighting lamp 1259 (FIG. 12) over lead 458; opens at its 4ACtA-2 break contact (FIG. 3) a connection to lead 363, and completes at its 4ACA-6 make contact (FIG. 3) another path for connection of ground to lead 311 to maintain answered super- Vision -to the calling panty.

Relay 4AC, released, connects at its 4AC-5, 4AC-6, 4AC-7 `and 4AC-8 break contacts (FIG. 3) the transmission path through to the links Vand interrupts at its 4AC-4 make contact (FIG. 4) the operating path of relay 4L1D, ywhich releases. Relay 4L1D, released, removes at its 4L1D-3 make contact (FIG. 4) battery from lead 412 whereby to extinguish pickup lamp 1213 (PU00) of the console circuit (FIG. 12).

Attendants Position and Console Circuits In addition to those functions ot the position and console circuits which have already been discussed above in connection with the originating and completing of a call, various other -functions are, of course, carried out by these circuits a few of which will be described at this point.

As pointed out previously relay 13S1G (FIG. 13) is operated during origination of a call and it was stated that operation of this relay caused an audible signal at the console to be operated. Relay 13S1G, openated, connects interrupted battery through interrupter contact IINT-4 (FIG. 12), ballast lamp 1251, make contact f13S1G-1 of relay 13S1G through Bell Chime 1252 which li-s sounded as an alerting signal to the attendant. When the attendant operates her Signal Cutoc Key 1380 (FIG. 13) -a path is completed for operation of relay 13SCO, traced from ground, make contact K-2 of relay 10CK, closed contact of signal cuto key 1380, make contact 13S1G-2 of relay 13S1G, break contact 13SCO2 of relay 13SCO, Winding of relay 13SCO to battery; relay 13SCO upon operating locks through its 13SCO-3 make contact to the ground on lead 363, releases relay 13S1G by interrupting the opera-ting path `at break contact 13SCO-1, and connects Vthrough its 13SCO-4 make contact (FG. 13) battery to the energizing paths of signal cutoff lamps 1353 and 1354 of the console whereby to light these lamps.

Relay 13S1G upon releasing interrupts at its 13S1G-1 make contact Ithe energizing path of Bell Chime 1252 which is silenced, and interrupts at its make Contact 13S1G-2 the path over which relay 13SCO was operated. Relay 13SCO remains operated over the locking path to lead 363, however, until all calls have been answered. The last line circuit to be answered removes ground from lead 363 yand relay 13SCO releases. Subsequent calls will operate relay 13S1G yas described.

Bell Chime 1252 (FIG. 12) may also be operated by closing 13ALM CONT-1 (FIG. 13) which completes an obvious path through break contact 13ACO-1 of relay 13ACO for operation of relay 13ALM; the path for energizing Bell Chime 1252 is then completed through make contact 13ALM-2 of relay 13ALM. ALM lamp 1357 of .thefconsole will light when 13ALM CONT-1 is closed.

`Closure of the Alarm Cutofr" key 1393 (FIG. 13) will operate relay 13ACO over `a path through break contact 13PH-1 of relay 13PH to the ISALM CONT-1 battery; relay 13ACO upon operating willlock to ground through its 13ACO-2 make contact. Relay 13ACO, operated, completes a path through its 13ACO-3 make contact for lighting Alarm Cutoi lamp 1358 (FIG. 13) yand opens at its. 13ACO-1 break contact the operating path of relay 13ALM, which releases. Relay 13ALM upon releasing interrupts at its 13ALM-2 make contact the energizing path of Bell Chime 1252 which is silenced. Relay 13ACO will` remain operated until the battery is removed at 13ALM CONT-1.

The console circuit telephone set, including transmitter 1289 and receiver 126), and the line circuits, as connected, are interconnected `on a four-wire basis by resistance, bridge 1271 which, preferably, follows the generalA arrangement covered by United States Patent 2,035,536, March 3l, 1936, F. A. Cowan et al. As disclosed in `that patent `a bridging network circuit comprises a balanced structure made up of resistances and having four equivalent branch outlets all arranged so that the impedance looking into -any branch is about 650 ohms. (In the vpresent instance only three of the branches are utilized and the fourthis not connected.) With this bridging arrangement there is a low-loss path through only tworesistors from any input to the other .two outputs. However, the only path `from an input to its own correspondingly numbered output is through several resistors in series and reversals are introduced in half of these paths whereby to produce -a 180 degree phase difference thereby greatly reducing the relection currents and resulting in very little interaction between the branch circuits. Amplification is provided by amplifiers 1272, 1273 and 1274 to overcome the loss introduced by the bridge.

Battery feed coils 1277 (FIG. l2) supply the transmitter; battery to the attendants telephone circuit while induction-,coil 1278 is utilized to match the transmitter impedance with that of `the line. Resistance pad 1281 is provided to control the level from the transmitter to the resista-nce bridge 1271. Resistors 11282 and 1233 provide desired 'sidetone paths and resistor 1234 provides termination. Resistors 1297, 1288, 1201 and 1202 are idle. circuit terminations for the lines.

When relay 13'1`M (FIG. 13) operates as previously described `following closure of the Talk Switch 1382, the battery supply path to transmitter 1289 (FIG. 12) is closed at make contact 1'3TM-2 and when relay 12H8 operates idle circuit termination resistor 11.284 is removed fromthe circuit at break contact -12HS2 and receiver 12 1260i is connected over make contact 12HS-1 to bridge 1271. Also, when relay :13TM is operated Talk lamps 131M- and 1317 (FIG. 13) are lighted over an obvious path through 13TM-3 make contact.

`Operation of the Signal Back Key 1387 (FIG. 13) of the console circuit, after a line has been picked up and the position circuit is in Talk condition, will operate relay 13SOR over an obvious path `from ground through make contact 13TlVl-1 of relay 13TM; this Will complete a path through make contact 13SOR-2 for lighting Signal Back lamps 13h15 and |1306 as an indication that the position circuit has functioned and the attendant may dial, :ring or automatic signal toward the calling party. Relay 138011 upon operating locks through its own make contact 13SOR-3, break contacts 13PDSS-2 of relay 13PDSS, 1381193 of relay 13STR, 1'3GS-3 of relay 13GS and 1391i of Link Release Switch, make contact 13TM-1 of relay 1 3TM to ground. Relay 1350K, operated, opens at its break contact 13SOR1 the locking path for relays 13STR and 13STR1, opens at its break contact 13SOR-4i the locking path -for relay 13DSSL and opens at its break contact y13SOR--5 a path over which ground is connected to coil 1223` (FIG. 12) through make contact 12B-1 of relay I12D when that relay is operated.

Operation of the Signal Forward Key 1386 (FIG. 13) of the console circuit, after a line has been picked up and the position circuit is in talk condition, will operate relay 13STR and .13STR1 from ground through make contact 13TM-1 of relay 13TM; `this completes a path through make contact 13STR-1 to light Signal Forward lamps 11328 and 1311 at the console whereby to indicate that the position circuit has functioned and that the attendant may dial, lring or automatic signal toward the called party. Relays 13STR and 13STR1, upon operating, lock to ground through make contact 13STR-2 of relay 13STR, break contacts 213SOR-1 of relay IBSOR, 13GS-3 of relay 13GS and `13941 of Link Release Key, make contact 13TM-1 of relay 13TM to ground. Relay 13STR, operated, opens at its ,13STR-3 break contact the locking path of relay 13SOR, opens at its 13STR-4 break contact the simplex between coils 1223 and 1214 (FIG. 12), connects ground `from the pulsing springs of calling dial I1217 of the console through break Contact 13FR-1 of relay 131FR and its own make contact 13STR-5 to coil 1214, and connects ground through its 13STR-6 make contact (FIG. 12) over lead '397 to the line pickup circuit. Relay 13STR1, operated, interrupts at its 13STR1-1 break contact (FIG. 13) the locking path of relay 13DSSL and removes at its `13STR1-2. break contact (FIG. 12) the connection of ground to coil 1223.

The Ring Key 1385 (FIG. '13) is utilized to ring, rering and' flash on a line picked by the attendant, the direction of the signal being determined by the position of the Signal Forward or Signal Back keys. Closure of the Ring key operates relay 13FR from ground through `131`M-1 malce Contact of relay 13T M; relay 13FR, operated, closes at its y13FR-2 make contact an energizing path for Ring lamps 1313 and 1331 (iFIG. 13) of the console.

Called Party Recalls Attendant After the originating, or calling party and the called party have been connected for talking and the attendant has disconnected as previously described above it may well happen that the called party may wish to recall the attendant, for `example to have the call transferred. Accordingly, the called subscriber will momentarily depress the called station switchhook which action is relayed from the called station to the calling station by annornentary opening of the transmitting line simplex. Relay 3S (FIG. 3) releases in response to this signal; relay 3S, released, releases relay SSA (make contact 3S-2) which, in turn, releases relay 41.51 (FIG. 4) by interrupting the holding path at make contact SSA-1. Relay 3LS2 (FIG.

3) will not release at this time si-nce, when relay 4LS1 releases, relay 3LS2 will then hold over a path to ground through break contacts 4LSI#6 of relay 4LS1 and `4L1D- 8 of relay 4L1D, make contact 3LL1-6 of relay 3LL1 and 3LS2-51 tot relay 3LS2. Relay 3S will again operate, following reclosure `or the called station loop (by releasing the switchhook), and operation of relay SSA follows 1operation of relay S. Relay 4LS1 (FIG 4) does not reoperate following reoperation of rel-ay 3SA, however, since its [operate path is open at break contact 3LS2-3 ot relay 3LS2. With relays SSA and SL82 operated and relay 4LS1 released a path is completed for ashi-ng destination lamp 1228 (FIG. 12) at 120 i.p.m. as a visual recall signal to the attendant. Lamp ,-1225 was previously lighted at steady by battery from make contact 4LS1-4 ot relay 4LSI but is now flashed by battery through interrupter contact INT-5 (FIG. 4) make contact SSA-2 of relay SSA, break contact 4LS1-7 of relay 4LS1, make contact SLSZ-fi or" relay 3LS2, lead 427, .lamp 1228 (FIG. l2) to battery. At the same time ground is applied to lead 363 (FIG. 3) through make contact 3LS2-6 of relay 3LS2, break contact 41431-8 of relay 4LS1 and make contact SSA-3 of relay lSSA; this ground operates relay ISSIG (FIG. 13) lof the position circuit, which in turn operates Bell Chime |1252 (FIG. 12) of the console circuit in the manner previously described.

The attendant in response to the audible and visual signals re-enters the line circuit involved by operating the particular pickup key associated with the dashing destination lamp. Operation of the pickup key is rollo-Wed by release of relay 4ACA (FIG. 4) -in the manner previously described above (the battery through resistor 44S being shunted by ground applied to lead *39S through break contact ISGS-2 of -relay 13GS, FIG. 12). Relay 4ACA, released, closes a path `for operation of relay iAC (FIG. 4) from the lground .lon lead 393 through break contacts y4H-3f of relay 4H, 4AC-2 of relay 4AC, 4ACA- 3 of relay 4ACA, winding of relay 4AC to battery; relay `4AC upon `operating locks Ito ground through its 4AC-3 make contact. Relay 4AC, operated, reoperates relay 4L1D (FIG. 4) through its make contact 4AC-4, and also splits the transmission paths at break contacts 4AC-5, !4AC-6, 4AC-7 and 4AC-8 (FIG. 3).

Relay 4L1D, operated, releases relay 3LS2 (FIG. 3) by opening the holding path therefor at break contact 4L1D-S; relay SL82, released, closes at its break contact 3LS2-3 a path over which relay `4LS1 (FIG. 4) now operates. Relay 4LS1, operated, closes at its 4LS1-3 make contact a path for operating relay SL82. Now, with relays 4LS1 and 3LS2 both operated, interrupted battery .is removed from line 427 (break contact 4LS1-7) and battery is connected steady thereto through make contacts 4LS1-4 of relay -4LS1 and 3LS2-4 of relay 3LS`2 whereby to :light destination lamp 1228 (FIG. 12) at steadyf Also, ground is removed rfrom lline 363 at break contact 4LS1-8 of relay `4LS1 (FIG. 3) whereby to release relay 13S1G (FIG. 13) and silence audible signal 1252 (FIG. l2) at the console. Since the transmission line has been split as above described the attendant can now talk with both the called and the calling party and can take what action is indicated.

Disconnect at Completion of Call Let us assume now that the conversation is completed and that the originating, or calling party `disconnects first. When the calling station goes -on-hook the opened loop on the transmit leads l251 and 252 releases relay 2L (FIG. 2). Relay 2L, released, releases in turn relay 2L1 by opening the operating path therefor at make contact 2L-1: Relay 2L1, released, removes at its 2L1-1 make contact the `ground from ylead 253 as an on-hook signal to the line circuit. Also, relay 2L1 by releasing interrupts at its 2L1-2 make contact the locking path for 14 relay ZAPLZ and opens at its 2LI-21 make contact the operating path of relay 2M). (Neither relay 2APL2 nor relay 2L@ releases at this point however.)

When the `ground is removed from lead 253y as -above described, relay SLL (FIG. 3) releases. Relay SLL, released, releases iat its 3LL-1 make contact relay 3LL1 which, in turn releases the other operated relays of the line circuit, Relay 4LI-I (FIG. 4) released at make contact 3LL1-5 of relay 3LL1, removes at make contact 4LII3 ground from lead y422 and the Line Hold Magnet in the Link and Connector Circuit (FIG. 5) permitting it to release. Also ground is removed at 4LH-4 make contact from lead 423 (FIG. 4) toward .the Link and Connector Circuit permitting the 4LH relay in the terminating line circuit (not shown but similar to originating `line circuit) to release, in turn releasing the line hold magnet for that circuit.

When the called party disconnects and opens the called station loop ground is removed fromJ the transmit line simplex releasing relay 3S (FIG. 3) in the originating line circuit. Relay 3S, released, releases relays SSA 'and 4LS1. Thus, with relay 3LS2 `operated and relays 4LS1 and .38A released battery (both steady and intermittent) is removed from lead 427 Vand destination lamp 1228 (FIG. 12) 1goes dark as an indication that the called party is on-hook. At this time also, that is when relay 3S releases, ground is removed from lead 311 at make contact .3S-1 (FIG. 3) rand relay ZAPL (FIG. 2) releases. Relay ZAPL, released, opens at its make contact 2APL-2 the operating path of relay ZRR and opens the holding path of relay ZAPLI at make contact 2APL--1. Relay 2AFL1 releases and opens Ithe operating path of relay 2APL2 at make con-tact 2APL1-3. Relay ZAPLZ released, opens at its 2APL2-5 make contact the holding path of relay 2L@ which releases. The circuit is now restored to normal condition.

Attendant Orginaled Outgoing Call The attendant in order to initiate a call to a station operates 4the pickup key (FIG. ll0) associated with the desired line. The line pickup circuit and the attendants position and consoie circuits operate in a manner similar to that -described above to connect the console to the desired line. When the console circuit is connected ground is connected to lead 398 at break contact ISGS-2 of relay 1368 (FIG. l2); this `ground applied to the line circuit over line 398 operates relay 4AC (FIG. 4) but shunts the battery connection through resistor 448 whereby Ato hold relay 4ACA nonoperated. Relay 4AC upon operating opens the path towards `line 39S at break contact 4AC-2, .locks to `gro-und through make contact 4AC-3, splits the transmission paths at break contacts 4AC-5, EAC-6, 4AC-l and 4AC- (FIG. 3) and completes at its vIAC-4 make contact (FIG. `4) a path for operating relay 4L1D'.

Relay 4L1D, operated, connects battery through its 4LID-3 make contact to lead 412 and thereover to light pickup lamp 1213 (FIG. 12), and connects battery through its 4LID-9 make contact (FIG. 4) to lead 461 and there-over to light lamp 1262 (FIG. 12) at the console circuit as a visual `busy signal. Also, relay 4L-1D 0perated connects through its 4L1D5 make contact ground over lead 414 towards marker 517 as a line busy mark.

Relay 3S (FIG. 3) operates `at this time from the simplex mark at the attendants position through break contact 3RV-4 of relay 3RV whereby to connect ground through its 3S-1 make contact (FIG. 3) over lead 311 to the connected auxiliary lline or two-Way trunk circuit to signal oit-hook supervision to that circuit. The connected .auxiliary line circuit or two-way trunk circuit then proceeds to signal lthe distant end in an appropriate manner Ias previously described.

Relay 3S, operated as above described, operates relays SSA (FIG. 3) and 4LS1 (FIG. 4) over paths previously described above, Battery is now applied to llead 

1. IN A TELEPHONE SYSTEM, A PRIVATE BRANCH EXCHANGE, AN ATTENDANT''S POSITION AT SAID PRIVATE BRANCH EXCHANGE, A PLURALITY OF STATION LINES, A FIRST CONFERENCE BRIDGE AND A SECOND CONFERENCE BRIDGE, BOTH OF SAID CONFERENCE BRIDGES BEING AT SAID SAME PRIVATE BRANCH EXCHANGE AND EACH OF SAID CONFERENCE BRIDGES HAVING A PLURALITY OF CONFERENCE LINES CONNECTED THERETO, MEANS UNDER CONTROL OF SAID ATTENDANT''S POSITION FOR CONNECTING SELECTED ONES OF SAID SATION LINES TO SELECTED ONES OF SAID CONFERENCE LINES CONNECTED TO SAID FIRST CONFERENCE BRIDGE EXCEPT FOR AN EXCLUSIVE ONE OF SAID CONFERENCE LINES AND FOR CONNECTING OTHER SELECTED ONES OF SAID STATION LINES TO SELECTED ONES OF SAID CONFERENCE LINES CONNECTED TO SAID SECOND CONFERENCE BRIDGE EXCEPT FOR AN EXCLUSIVE ONE OF SAID CONFERENCEE LINES, AND MEANS ALSO UNDER CONTROL OF SAID ATTENDANT''S POSITION FOR CONNECTING SAID EXCLUSIVE ONES OF SAID CONFERENCE LINES TOGETHER TO CONNECT SAID CONFERENCE BRIDGES IN TANDEM WHEREBY TO PROVIDE A SINGLE CONFERENCE LOOP OF RELATIVELY LARGE CAPACITY. 